A ballast provides power to a lamp and regulates the current and/or power provided to the lamp. When a lamp (e.g. a fluorescent lamp) nears the end of its usable life or breaks, the resistance of the lamp increases as seen by the ballast. The increased resistance requires the ballast to output higher voltages in order to maintain the current or power transferred to the lamp. Thus, the ballast develops very high voltages (e.g., voltages in excess of 500 volts AC) as the resistance continues to increase. The high voltage poses an electrocution hazard to a technician who needs to replace the old lamp because the increased voltage increases the risk that the electricity will arc to earth ground through the technician as he attempts to replace the lamp. Therefore, some ballasts are equipped with a protection circuit (e.g., an end of lamp life circuit) to prevent high voltage from being provided to the lamp. The protection circuit is configured to detect sudden increases in output voltage and/or output voltages in excess of a threshold value to shut down the ballast operation in response thereto. These ballasts may also have a circuit configured to detect when a lamp has been replaced and to restart the high voltage output of the ballast in response thereto in order to light the replacement lamp (e.g., by resetting the end of lamp life circuit).
A ballast may receive power from multiple sources. For example, ballast systems used in commercial buildings commonly receive power from a utility line supply and from a battery. Such a ballast system includes a primary ballast which provides a lamp with power when the ballast system is operating in a first operation mode (e.g., primary power mode) and a battery powered ballast (broadly, “backup ballast”) which provides the lamp with power when the ballast system is operating in a second operation mode (e.g., emergency power mode). The ballast system may include a switching circuit for controlling the operating mode of the ballast system. In particular, the switching circuit is configured to operate the ballast system in the primary power mode when the utility line supply is providing power to the ballast system and to operate the ballast system in the emergency power mode when the utility line supply is not providing power to the ballast system. Accordingly, when the utility line supply is providing power to the ballast system, the primary ballast provides the lamp with the power being supplied by the utility line supply. When the utility line stops providing power to the ballast system (e.g., during a power outage), the backup ballast provides the lamp with power supplied from the battery.
When the ballast system switches between the power sources, changes in the output voltage often occur causing the protection circuit to unnecessarily shut down ballast system operations. For example, the switching circuit generally responds to an interruption of the utility line supply power by immediately switching from the primary power mode to the emergency power mode. As a result, the backup ballast may begin providing power to the lamps before the primary ballast has been properly discharged. The excess output voltage that is discharged from the primary ballast may cause the protection circuit to shut down the output of the primary ballast.
FIG. 1 is a timing diagram for a conventional ballast system which illustrates the responses of components of the ballast system to a power outage event. The primary ballast of the ballast system includes a converter for converting AC (alternating current) voltage received from the utility line supply into a DC (direct current voltage) voltage. The DC voltage is then passed through a filtering capacitor to an inverter. The inverter converts the DC voltage into high frequency AC power for providing to the lamps. A voltage across the filtering capacitor (i.e., DC rail) may be present after the utility line supply is shut off while the filtering capacitor dissipates. Accordingly, as illustrated by the timing diagram in FIG. 1, the inverter remains on for a period of time, denoted tD, after the switching circuit has begun operating the ballast system in the emergency power mode (e.g., after the switching circuit is been shut off). The excess voltage output by the inverter during tD may trigger the protective circuit (e.g., end of lamp life circuit of the primary ballast) to erroneously detect that the lamps are broken or at the end of their useful lives and shut down the output of the primary ballast. As such, when power is restored to the ballast system via the utility line supply, the ballast system fails to provide power to the lamp since the output of the primary ballast is shut down.